A Solo With RTK

Mar 13, 2022
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I finally hacked a Solo for RTK, after proving the concept on the bench. The goal was to have a RTL event land the Solo on the 2' square launch pad.

I had on hand two Sparkfun RTK breakout boards from a previous project involving an IRIS+. I also have a state run NTRIP outlet 1 km away, so no problem with RTCM3 corrections. The rover receiver is mounted with Velcro in the gimbal bay. The original rat Rev. A GPS was removed, and the cable modified to terminate in Duponts, as the F9P board was already staked-out.
The antenna is critical for RTK - I had a decent L1 / L2 magnetic patch on hand with a 3m cable. A proper ground plane for L1/L2 was cut from sheet copper, and a thin piece of steel, cut from a Mason jar lid, was soldered in the center. This is centered on a Solo battery with more Velcro, and two 1" square pads of DuBro antivibe foam
were pressed between the battery and ground plane to (hopefully) kill the vibes. It worked, as all vibes came in under 8.
The 3m cable was bundled and secured under Solo - taped for test purposes. It isn't advisable to shorten GNSS RX cables as the receiver may be overdriven and adversely impacted. Clearly not a long term fix - PIX below.
Mission Planner was used to emit the corrections. After connecting MP to sololink, the RTK insert page (Advanced Setup tab) allows selection of the correction source.
With my local NTRIP source, the Solo achieved the FLOAT instantly, and FIX after two minutes. The Sparkfun F9P was locked on 18 birds at the time, at 0.8 hdop.

The second F9P was configured in the standard fashion to serve as a base. I have a decent survey antenna for that. After a quick site survey to 0.5m I switched MP to accept corrections from the base COM port. Again, a FIX was obtained.

I was able to get one decent test flight in due to rotten weather. RTK is as advertised- rock solid hover in loiter - it literally sat there. However, the landing missed the square, so have work to do. (The miss was ~1' in front of the pad...)

I need to add that RTK FIX IS possible indoors in some cases. I live in a 100% stickbuilt house and get almost as good GNSS reception on my bench as I get with a proper survey antenna, outside of course. I was shocked, however!

The Solo turned out to be a good test platform, due to sololink WiFi. The correction stream from MP is seamlessly inserted and directed to the F9P, eliminating the need for a secondary delivery method like SIK radios, etc. The F9P uses its UART1 for all communication with Arducopter, and is configured to accept and forward RTCM3 corrections.

This is purely a science project and not intended for serious work - minus the tape, Dupont connectors, Velcro, etc. Having Solo fun!
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